Insulators are used to prevent the loss of electric charge or current from conductors in electric power transmission lines. A typical insulator is made from a material which has a very high resistance to electric current, so that the current flow through it is usually negligible. One type of insulator is referred to as a suspension insulator. It suspends a transmission line from an overhead support. A particular type of suspension insulator is referred to as a composite insulator.
A known composite insulator may include a coated fiberglass rod surrounded by weathersheds made from a highly insulating material. (See, for example, Kuhl U.S. Pat. No. 4,217,466, or, Bauer U.S. Pat. No. 4,246,696). Such insulators are referred to as "composite" because the weathersheds and the rod are made from different materials especially suited for the distinct functions of the two components. In general, the rod takes the mechanical stress and contributes to the overall insulation, while the majority of the insulation is provided by the weathersheds.
Another type of insulator is referred to as a guy strain insulator. It provides insulation for guy wires attached to the transmission line's support structure. One of the requirements for guy strain insulators is that they be capable of providing torque resistance.
Insulators may be connected to and carry the power line in a variety of ways, including "suspension," "deadend," "horizontal" and "post" configurations. In all of these configurations, metal fittings are provided at each end of the rod for connecting the rod to either a support pylon or a power line. Numerous U.S. Patents disclose structure and methods for mounting such fittings on an insulator. A partial list of such patents includes Wheeler U.S. Pat. No. 4,505,033; Kaczerginski et al. U.S. Pat. No. 4,491,687; Dougherty U.S. Pat. No. 3,592,959; Von Platen U.S. Pat. No. 2,970,186; Hocks U.S. Pat. No. 3,134,164; Ely U.S. Pat. No. 3,878,321; Austin et al. U.S. Pat. No. 3,014,088; Rebosio U.S. Pat. No. 3,971,128; Vose U.S. Pat. No. 3,328,515; Ishihara et al. U.S. Pat. No. 4,296,276; Harmon U.S. Pat. No. 3,586,758; Willem U.S. Pat. No. 4,057,687; Lusk U.S. Pat. No. 4,198,538; Kalb U.S. Pat. No. 3,898,372; Thuillier et al. U.S. Pat. No. 4,423,285; and Winkler et al. U.S. Pat. No. 4,373,113.
Wheeler U.S. Pat. No. 4,505,033 discloses a composite insulator comprising a fiberglass rod partially covered with an elastomer sheath. A metal fitting is mounted on the exposed end of the rod for connecting the insulator to associated members. The fitting includes a head formed with an oval eye for attachment to a support or to a high voltage conductor. The fitting may be adhesive bonded to the rod, or it may be swaged to the rod.
Kaczerginski U.S. Pat. No. 4,491,687 discloses a composite insulator comprising a central mandrel made of insulating foam with two metal end pieces glued to the mandrel. Resin-impregnated fibers are wound around the end pieces and mandrel. The winding is then covered with a resilient, insulating covering.
Dougherty U.S. Pat. No. 3,592,959 discloses a prestressed, ceramic insulator assembly including an endless fiberglass loop. A return bend portion of the loop is supported in a U-shaped saddle member, which is in alignment with an end cap.
Von Platen U.S. Pat. No. 2,970,186 discloses an electric insulator including a member made from thin vitreous wires. The member is wound in a closed loop, the ends of which rest against the corresponding contact surfaces of two separate parts. Each of these parts is screwed into a separate end cap.
There are a number of problems associated with many such known insulators. For example, the fittings are often relatively large components cast from metal and, thus, are heavy and expensive. Where a fitting is swaged to a fiberglass portion of an insulator, fabrication is difficult because it is difficult to apply the correct amount of pressure to the metal, which is ductile, without shattering the glass, which is considerably less ductile. Where fiberglass is glued to metal, it is difficult to find a suitable adhesive which will join the metal-resin-glass interface and not break down during use.